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			<h1>Java String 类</h1>
<p>
字符串广泛应用 在 Java 编程中，在 Java 中字符串属于对象，Java 提供了 String 类来创建和操作字符串。
</p><hr>
<h2>创建字符串</h2>
<p>创建字符串最简单的方式如下:</p>
<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">str</span><span class="hl-code"> = </span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-code">;</span></div>
</div>
</div>
<p>
在代码中遇到字符串常量时，这里的值是 "<strong>Runoob</strong>"，编译器会使用该值创建一个 String 对象。
</p><p>和其它对象一样，可以使用关键字和构造方法来创建 String 对象。
<p>用构造函数创建字符串：</p>

<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">str2</span><span class="hl-code">=</span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">String</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<p>String 创建的字符串存储在公共池中，而 new 创建的字符串对象在堆上：</p>
<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">s1</span><span class="hl-code"> = </span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-code">;              </span><span class="hl-comment">//</span><span class="hl-comment"> String 直接创建</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">s2</span><span class="hl-code"> = </span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-code">;              </span><span class="hl-comment">//</span><span class="hl-comment"> String 直接创建</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">s3</span><span class="hl-code"> = </span><span class="hl-identifier">s1</span><span class="hl-code">;                    </span><span class="hl-comment">//</span><span class="hl-comment"> 相同引用</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">s4</span><span class="hl-code"> = </span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">String</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;   </span><span class="hl-comment">//</span><span class="hl-comment"> String 对象创建</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">s5</span><span class="hl-code"> = </span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">String</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">Runoob</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;   </span><span class="hl-comment">//</span><span class="hl-comment"> String 对象创建</span><span class="hl-comment"></span></div>
</div>
</div>
<p><img decoding="async" src="https://www.runoob.com/wp-content/uploads/2013/12/java-string-1-2020-12-01.png"></p>
</p>

<p>String 类有 11 种构造方法，这些方法提供不同的参数来初始化字符串，比如提供一个字符数组参数:</p>
<div class="example">
<h2 class="example">StringDemo.java 文件代码：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-reserved">class</span><span class="hl-code"> </span><span class="hl-identifier">StringDemo</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-types">static</span><span class="hl-code"> </span><span class="hl-types">void</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">args</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-brackets">)</span><span class="hl-brackets">{</span><span class="hl-code">
      </span><span class="hl-types">char</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-code"> </span><span class="hl-identifier">helloArray</span><span class="hl-code"> = </span><span class="hl-brackets">{</span><span class="hl-code"> </span><span class="hl-quotes">'</span><span class="hl-string">r</span><span class="hl-quotes">'</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">u</span><span class="hl-quotes">'</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">n</span><span class="hl-quotes">'</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">o</span><span class="hl-quotes">'</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">o</span><span class="hl-quotes">'</span><span class="hl-code">, </span><span class="hl-quotes">'</span><span class="hl-string">b</span><span class="hl-quotes">'</span><span class="hl-brackets">}</span><span class="hl-code">;
      </span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">helloString</span><span class="hl-code"> = </span><span class="hl-reserved">new</span><span class="hl-code"> </span><span class="hl-identifier">String</span><span class="hl-brackets">(</span><span class="hl-identifier">helloArray</span><span class="hl-brackets">)</span><span class="hl-code">;  
      </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-identifier">helloString</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
   </span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>以上实例编译运行结果如下：</p>
<pre>
runoob
</pre>
<p>
<strong>注意:</strong>String 类是不可改变的，所以你一旦创建了 String 对象，那它的值就无法改变了（详看笔记部分解析）。</p><p>
如果需要对字符串做很多修改，那么应该选择使用 <a href="../java/java-stringbuffer.html" target="_blank" title="StringBuffer & StringBuilder 类" rel="noopener noreferrer">StringBuffer & StringBuilder 类</a>。
</p><hr>
<h2>字符串长度</h2>
<p>
</p><p>用于获取有关对象的信息的方法称为访问器方法。
</p><p>String 类的一个访问器方法是 length() 方法，它返回字符串对象包含的字符数。
</p><p>下面的代码执行后，len 变量等于 14:</p>
<div class="example">
<h2 class="example">StringDemo.java 文件代码：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-reserved">class</span><span class="hl-code"> </span><span class="hl-identifier">StringDemo</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-types">static</span><span class="hl-code"> </span><span class="hl-types">void</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">args</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
        </span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">site</span><span class="hl-code"> = </span><span class="hl-quotes">&quot;</span><span class="hl-string">www.runoob.com</span><span class="hl-quotes">&quot;</span><span class="hl-code">;
        </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">len</span><span class="hl-code"> = </span><span class="hl-identifier">site</span><span class="hl-code">.</span><span class="hl-identifier">length</span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">;
        </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-quotes">&quot;</span><span class="hl-string">菜鸟教程网址长度 : </span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">len</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">;
   </span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>以上实例编译运行结果如下：</p>
<pre>
菜鸟教程网址长度 : 14
</pre><hr>
<h2>连接字符串</h2>
<p>String 类提供了连接两个字符串的方法：</p>
<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">string1</span><span class="hl-code">.</span><span class="hl-identifier">concat</span><span class="hl-brackets">(</span><span class="hl-identifier">string2</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<p>
返回 string2 连接 string1 的新字符串。也可以对字符串常量使用 concat() 方法，如：
</p>
<pre>"我的名字是 ".concat("Runoob");</pre>
<p>
更常用的是使用'+'操作符来连接字符串，如：</p>
<pre>
"Hello," + " runoob" + "!"</pre>
<p>
结果如下:</p>
<pre>
"Hello, runoob!"</pre>
<p>
下面是一个例子:</p>
<div class="example">
<h2 class="example">StringDemo.java 文件代码：</h2> 
<div class="example_code">
<div class="hl-main"><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-reserved">class</span><span class="hl-code"> </span><span class="hl-identifier">StringDemo</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">
    </span><span class="hl-reserved">public</span><span class="hl-code"> </span><span class="hl-types">static</span><span class="hl-code"> </span><span class="hl-types">void</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-brackets">(</span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">args</span><span class="hl-brackets">[</span><span class="hl-brackets">]</span><span class="hl-brackets">)</span><span class="hl-code"> </span><span class="hl-brackets">{</span><span class="hl-code">     
        </span><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">string1</span><span class="hl-code"> = </span><span class="hl-quotes">&quot;</span><span class="hl-string">菜鸟教程网址：</span><span class="hl-quotes">&quot;</span><span class="hl-code">;     
        </span><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">println</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">1、</span><span class="hl-quotes">&quot;</span><span class="hl-code"> + </span><span class="hl-identifier">string1</span><span class="hl-code"> + </span><span class="hl-quotes">&quot;</span><span class="hl-string">www.runoob.com</span><span class="hl-quotes">&quot;</span><span class="hl-brackets">)</span><span class="hl-code">;  
    </span><span class="hl-brackets">}</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>以上实例编译运行结果如下：</p>
<pre>
1、菜鸟教程网址：www.runoob.com
</pre>
<hr>
<h2>创建格式化字符串</h2>
<p>我们知道输出格式化数字可以使用 printf() 和 format() 方法。</p><p>String 类使用静态方法 format() 返回一个String 对象而不是 PrintStream 对象。
</p><p>String 类的静态方法 format() 能用来创建可复用的格式化字符串，而不仅仅是用于一次打印输出。</p><p>如下所示：
</p>
<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">System</span><span class="hl-code">.</span><span class="hl-identifier">out</span><span class="hl-code">.</span><span class="hl-identifier">printf</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">浮点型变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                  </span><span class="hl-quotes">&quot;</span><span class="hl-string">%f, 整型变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                  </span><span class="hl-quotes">&quot;</span><span class="hl-string"> %d, 字符串变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                  </span><span class="hl-quotes">&quot;</span><span class="hl-string">is %s</span><span class="hl-quotes">&quot;</span><span class="hl-code">, </span><span class="hl-identifier">floatVar</span><span class="hl-code">, </span><span class="hl-identifier">intVar</span><span class="hl-code">, </span><span class="hl-identifier">stringVar</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<p>你也可以这样写</p>
<div class="example">
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">String</span><span class="hl-code"> </span><span class="hl-identifier">fs</span><span class="hl-code">;
</span><span class="hl-identifier">fs</span><span class="hl-code"> = </span><span class="hl-identifier">String</span><span class="hl-code">.</span><span class="hl-identifier">format</span><span class="hl-brackets">(</span><span class="hl-quotes">&quot;</span><span class="hl-string">浮点型变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                   </span><span class="hl-quotes">&quot;</span><span class="hl-string">%f, 整型变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                   </span><span class="hl-quotes">&quot;</span><span class="hl-string"> %d, 字符串变量的值为 </span><span class="hl-quotes">&quot;</span><span class="hl-code"> +
                   </span><span class="hl-quotes">&quot;</span><span class="hl-string"> %s</span><span class="hl-quotes">&quot;</span><span class="hl-code">, </span><span class="hl-identifier">floatVar</span><span class="hl-code">, </span><span class="hl-identifier">intVar</span><span class="hl-code">, </span><span class="hl-identifier">stringVar</span><span class="hl-brackets">)</span><span class="hl-code">;</span></div>
</div>
</div>
<hr>
<h2>String 方法</h2>
<p>下面是 String 类支持的方法，更多详细，参看 <a href="../manual/jdk11api/java.base/java/lang/String.html" target="_blank" rel="noopener noreferrer">Java String API</a> 文档:</p>
<table class="reference">
	<tbody>
		<tr>
			<th style="width:76px;">
				SN(序号)</th>
			<th style="width:501px;">
				方法描述</th>
		</tr>
		<tr>
			<td style="width:76px;">
				1</td>
			<td style="width:501px;">
				<a href="java-string-charat.html" target="_blank" rel="noopener noreferrer">char charAt(int index)</a><br>
				返回指定索引处的 char 值。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				2</td>
			<td style="width:501px;">
				<a href="java-string-compareto.html" target="_blank" rel="noopener noreferrer">int compareTo(Object o)</a><br>
				把这个字符串和另一个对象比较。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				3</td>
			<td style="width:501px;">
				<a href="java-string-compareto.html" target="_blank" rel="noopener noreferrer">int compareTo(String anotherString)</a><br>
				按字典顺序比较两个字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				4</td>
			<td style="width:501px;">
				<a href="java-string-comparetoignorecase.html" target="_blank" rel="noopener noreferrer">int compareToIgnoreCase(String str)</a><br>
				按字典顺序比较两个字符串，不考虑大小写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				5</td>
			<td style="width:501px;">
				<a href="java-string-concat.html" target="_blank" rel="noopener noreferrer">String concat(String str)</a><br>
				将指定字符串连接到此字符串的结尾。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				6</td>
			<td style="width:501px;">
				<a href="java-string-contentequals.html" target="_blank" rel="noopener noreferrer">boolean contentEquals(StringBuffer sb)</a><br>
				当且仅当字符串与指定的StringBuffer有相同顺序的字符时候返回真。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				7</td>
			<td style="width:501px;">
				<a href="java-string-copyvalueof.html" target="_blank" rel="noopener noreferrer">static String copyValueOf(char[] data)</a><br>
				返回指定数组中表示该字符序列的 String。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				8</td>
			<td style="width:501px;">
				<a href="java-string-copyvalueof.html" target="_blank" rel="noopener noreferrer">static String copyValueOf(char[] data, int offset, int count)</a><br>
				返回指定数组中表示该字符序列的 String。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				9</td>
			<td style="width:501px;">
				<a href="java-string-endswith.html" target="_blank" rel="noopener noreferrer">boolean endsWith(String suffix)</a><br>
				测试此字符串是否以指定的后缀结束。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				10</td>
			<td style="width:501px;">
				<a href="java-string-equals.html" target="_blank" rel="noopener noreferrer">boolean equals(Object anObject)</a><br>
				将此字符串与指定的对象比较。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				11</td>
			<td style="width:501px;">
				<a href="java-string-equalsignorecase.html" target="_blank" rel="noopener noreferrer">boolean equalsIgnoreCase(String anotherString)</a><br>
				将此 String 与另一个 String 比较，不考虑大小写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				12</td>
			<td style="width:501px;">
				<a href="java-string-getbytes.html" target="_blank" rel="noopener noreferrer">byte[] getBytes()</a><br>
				&nbsp;使用平台的默认字符集将此 String 编码为 byte 序列，并将结果存储到一个新的 byte 数组中。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				13</td>
			<td style="width:501px;">
				<a href="java-string-getbytes.html" target="_blank" rel="noopener noreferrer">byte[] getBytes(String charsetName)</a><br>
				使用指定的字符集将此 String 编码为 byte 序列，并将结果存储到一个新的 byte 数组中。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				14</td>
			<td style="width:501px;">
				<a href="java-string-getchars.html" target="_blank" rel="noopener noreferrer">void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)</a><br>
				将字符从此字符串复制到目标字符数组。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				15</td>
			<td style="width:501px;">
				<a href="java-string-hashcode.html" target="_blank" rel="noopener noreferrer">int hashCode()</a><br>
				返回此字符串的哈希码。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				16</td>
			<td style="width:501px;">
				<a href="java-string-indexof.html" target="_blank" rel="noopener noreferrer">int indexOf(int ch)</a><br>
				返回指定字符在此字符串中第一次出现处的索引。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				17</td>
			<td style="width:501px;">
				<a href="java-string-indexof.html" target="_blank" rel="noopener noreferrer">int indexOf(int ch, int fromIndex)</a><br>
				返回在此字符串中第一次出现指定字符处的索引，从指定的索引开始搜索。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				18</td>
			<td style="width:501px;">
				<a href="java-string-indexof.html" target="_blank" rel="noopener noreferrer">int indexOf(String str)</a><br>
				&nbsp;返回指定子字符串在此字符串中第一次出现处的索引。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				19</td>
			<td style="width:501px;">
				<a href="java-string-indexof.html" target="_blank" rel="noopener noreferrer">int indexOf(String str, int fromIndex)</a><br>
				返回指定子字符串在此字符串中第一次出现处的索引，从指定的索引开始。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				20</td>
			<td style="width:501px;">
				<a href="java-string-intern.html" target="_blank" rel="noopener noreferrer">String intern()</a><br>
				&nbsp;返回字符串对象的规范化表示形式。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				21</td>
			<td style="width:501px;">
				<a href="java-string-lastindexof.html" target="_blank" rel="noopener noreferrer">int lastIndexOf(int ch)</a><br>
				&nbsp;返回指定字符在此字符串中最后一次出现处的索引。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				22</td>
			<td style="width:501px;">
				<a href="java-string-lastindexof.html" target="_blank" rel="noopener noreferrer">int lastIndexOf(int ch, int fromIndex)</a><br>
				返回指定字符在此字符串中最后一次出现处的索引，从指定的索引处开始进行反向搜索。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				23</td>
			<td style="width:501px;">
				<a href="java-string-lastindexof.html" target="_blank" rel="noopener noreferrer">int lastIndexOf(String str)</a><br>
				返回指定子字符串在此字符串中最右边出现处的索引。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				24</td>
			<td style="width:501px;">
				<a href="java-string-lastindexof.html" target="_blank" rel="noopener noreferrer">int lastIndexOf(String str, int fromIndex)</a><br>
				&nbsp;返回指定子字符串在此字符串中最后一次出现处的索引，从指定的索引开始反向搜索。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				25</td>
			<td style="width:501px;">
				<a href="java-string-length.html" target="_blank" rel="noopener noreferrer">int length()</a><br>
				返回此字符串的长度。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				26</td>
			<td style="width:501px;">
				<a href="java-string-matches.html" target="_blank" rel="noopener noreferrer">boolean matches(String regex)</a><br>
				告知此字符串是否匹配给定的正则表达式。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				27</td>
			<td style="width:501px;">
				<a href="java-string-regionmatches.html" target="_blank" rel="noopener noreferrer">boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)</a><br>
				测试两个字符串区域是否相等。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				28</td>
			<td style="width:501px;">
				<a href="java-string-regionmatches.html" target="_blank" rel="noopener noreferrer">boolean regionMatches(int toffset, String other, int ooffset, int len)</a><br>
				测试两个字符串区域是否相等。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				29</td>
			<td style="width:501px;">
				<a href="java-string-replace.html" target="_blank" rel="noopener noreferrer">String replace(char oldChar, char newChar)</a><br>
				返回一个新的字符串，它是通过用 newChar 替换此字符串中出现的所有 oldChar 得到的。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				30</td>
			<td style="width:501px;">
				<a href="java-string-replaceall.html" target="_blank" rel="noopener noreferrer">String replaceAll(String regex, String replacement)</a><br>
				使用给定的 replacement 替换此字符串所有匹配给定的正则表达式的子字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				31</td>
			<td style="width:501px;">
				<a href="java-string-replacefirst.html" target="_blank" rel="noopener noreferrer">String replaceFirst(String regex, String replacement)</a><br>
				&nbsp;使用给定的 replacement 替换此字符串匹配给定的正则表达式的第一个子字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				32</td>
			<td style="width:501px;">
				<a href="java-string-split.html" target="_blank" rel="noopener noreferrer">String[] split(String regex)</a><br>
				根据给定正则表达式的匹配拆分此字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				33</td>
			<td style="width:501px;">
				<a href="java-string-split.html" target="_blank" rel="noopener noreferrer">String[] split(String regex, int limit)</a><br>
				根据匹配给定的正则表达式来拆分此字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				34</td>
			<td style="width:501px;">
				<a href="java-string-startswith.html" target="_blank" rel="noopener noreferrer">boolean startsWith(String prefix)</a><br>
				测试此字符串是否以指定的前缀开始。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				35</td>
			<td style="width:501px;">
				<a href="java-string-startswith.html" target="_blank" rel="noopener noreferrer">boolean startsWith(String prefix, int toffset)</a><br>
				测试此字符串从指定索引开始的子字符串是否以指定前缀开始。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				36</td>
			<td style="width:501px;">
				<a href="java-string-subsequence.html" target="_blank" rel="noopener noreferrer">CharSequence subSequence(int beginIndex, int endIndex)</a><br>
				&nbsp;返回一个新的字符序列，它是此序列的一个子序列。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				37</td>
			<td style="width:501px;">
				<a href="java-string-substring.html" target="_blank" rel="noopener noreferrer">String substring(int beginIndex)</a><br>
				返回一个新的字符串，它是此字符串的一个子字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				38</td>
			<td style="width:501px;">
				<a href="java-string-substring.html" target="_blank" rel="noopener noreferrer">String substring(int beginIndex, int endIndex)</a><br>
				返回一个新字符串，它是此字符串的一个子字符串。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				39</td>
			<td style="width:501px;">
				<a href="java-string-tochararray.html" target="_blank" rel="noopener noreferrer">char[] toCharArray()</a><br>
				将此字符串转换为一个新的字符数组。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				40</td>
			<td style="width:501px;">
				<a href="java-string-tolowercase.html" target="_blank" rel="noopener noreferrer">String toLowerCase()</a><br>
				使用默认语言环境的规则将此 String 中的所有字符都转换为小写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				41</td>
			<td style="width:501px;">
				<a href="java-string-tolowercase.html" target="_blank" rel="noopener noreferrer">String toLowerCase(Locale locale)</a><br>
				&nbsp;使用给定 Locale 的规则将此 String 中的所有字符都转换为小写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				42</td>
			<td style="width:501px;">
				<a href="java-string-tostring.html" target="_blank" rel="noopener noreferrer">String toString()</a><br>
				&nbsp;返回此对象本身（它已经是一个字符串！）。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				43</td>
			<td style="width:501px;">
				<a href="java-string-touppercase.html" target="_blank" rel="noopener noreferrer">String toUpperCase()</a><br>
				使用默认语言环境的规则将此 String 中的所有字符都转换为大写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				44</td>
			<td style="width:501px;">
				<a href="java-string-touppercase.html" target="_blank" rel="noopener noreferrer">String toUpperCase(Locale locale)</a><br>
				使用给定 Locale 的规则将此 String 中的所有字符都转换为大写。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				45</td>
			<td style="width:501px;">
				<a href="java-string-trim.html" target="_blank" rel="noopener noreferrer">String trim()</a><br>
				返回字符串的副本，忽略前导空白和尾部空白。</td>
		</tr>
		<tr>
			<td style="width:76px;">
				46</td>
			<td style="width:501px;">
				<a href="java-string-valueof.html" target="_blank" rel="noopener noreferrer">static String valueOf(primitive data type x)</a><br>
				返回给定data type类型x参数的字符串表示形式。</td>
		</tr>
<tr>
			<td style="width:76px;">
				47</td>
			<td style="width:501px;">
				<a href="java-string-contains.html" target="_blank" rel="noopener noreferrer">contains(CharSequence chars)</a><br>
				判断是否包含指定的字符系列。</td>
		</tr>
<tr>
			<td style="width:76px;">
				48</td>
			<td style="width:501px;">
				<a href="java-string-isempty.html" target="_blank" rel="noopener noreferrer">isEmpty()</a><br>
				判断字符串是否为空。</td>
		</tr>
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<div id="postcomments"  style="display:none;" >
	<ol class="commentlist">
		<li class="comment byuser comment-author-tianqixin bypostauthor even thread-even depth-1" id="comment-1044"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;tianqixin</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;429***967@qq.com</p></div><div id="runoobvote-id-1044" data-commid = "1044" class="upvotejs"><a class="upvote"></a> <span class="count">355</span></div></div><div class="comt-main" id="div-comment-1044"><p data-title="java String 类为什么是不可改变的？" data-commid="25487">String 类是不可改变的解析，例如：</p>
<pre>
String s = "Google";
System.out.println("s = " + s);

s = "Runoob";
System.out.println("s = " + s);
</pre>
<p>输出结果为：</p>
<pre>
Google
Runoob
</pre>
<p>从结果上看是改变了，但为什么说 String 对象是不可变的呢？</p>
<p>原因在于实例中的 <b>s</b> 只是一个 <b>String</b> 对象的引用，并不是对象本身，当执行 <b>s = "Runoob";</b> 创建了一个新的对象 "Runoob"，而原来的 "Google" 还存在于内存中。</p>
<p><img src="//www.runoob.com/wp-content/uploads/2013/12/string-no-modify.png"></p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/1044">tianqixin</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;tianqixin</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;429***967@qq.com</p></div></span>6年前 (2017-02-27)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-20596"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;豆妮的小奴隶</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;861***743@qq.com</p></div><div id="runoobvote-id-20596" data-commid = "20596" class="upvotejs"><a class="upvote"></a> <span class="count">123</span></div></div><div class="comt-main" id="div-comment-20596"><p data-title="Java length() 方法，length 属性和 size() 方法的区别">length() 方法，length 属性和 size() 方法的区别:</p>
<ul><li>
1、<span class="marked">length() </span>方法是针对字符串来说的，要求一个字符串的长度就要用到它的length()方法；
</li><li>
2、<span class="marked">length 属性</span>是针对 Java 中的数组来说的，要求数组的长度可以用其 length 属性；
</li><li>
3、Java 中的 <span class="marked">size()</span> 方法是针对泛型集合说的, 如果想看这个泛型有多少个元素, 就调用此方法来查看!
</li></ul>
<p>这个例子来演示这两个方法和一个属性的用法：</p>
<pre>
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        String array[] = { "First", "Second", "Third" };
        String a = "HelloWorld";
        List&lt;String&gt; list = new ArrayList&lt;String&gt;();
        list.add(a);
        System.out.println("数组array的长度为" + array.length);
        System.out.println("字符串a的长度为" + a.length());
        System.out.println("list中元素个数为" + list.size());

    }
}
</pre>
<p>
输出的值为:
</p><pre>
数组array的长度为3
字符串a的长度为10
list中元素个数为1</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/20596">豆妮的小奴隶</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;豆妮的小奴隶</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;861***743@qq.com</p></div></span>6年前 (2017-11-21)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-20810"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Helen</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;109***2166@qq.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="http://soft.chinabyte.com/database/182/12171182.shtml">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-20810" data-commid = "20810" class="upvotejs"><a class="upvote"></a> <span class="count">54</span></div></div><div class="comt-main" id="div-comment-20810"><p data-titile="JAVA String.format 方法使用介绍"><strong>1.对整数进行格式化：%[index$][标识][最小宽度]转换方式</strong></p>

<p>格式化字符串由4部分组成，特殊的格式常以%index$开头，index从1开始取值，表示将第index个参数拿进来进行格式化，[最小宽度]的含义也很好理解，就是最终该整数转化的字符串最少包含多少位数字。剩下2个部分的含义：</p>

<p>标识：</p>
<ul><li>
'-' 在最小宽度内左对齐，不可以与"用0填充"同时使用
</li><li>
'#' 只适用于8进制和16进制，8进制时在结果前面增加一个0，16进制时在结果前面增加0x
</li><li>
'+' 结果总是包括一个符号(一般情况下只适用于10进制，若对象为BigInteger才可以用于8进制和16进制)
</li><li>
' ' 正值前加空格，负值前加负号(一般情况下只适用于10进制，若对象为BigInteger才可以用于8进制和16进制)
</li><li>
'0' 结果将用零来填充
</li><li>
',' 只适用于10进制，每3位数字之间用"，"分隔
</li><li>
'(' 若参数是负数，则结果中不添加负号而是用圆括号把数字括起来(同'+'具有同样的限制)
</li></ul>
<p>转换方式：</p>

<p>d-十进制  o-八进制  x或X-十六进制</p>

<p>上面的说明过于枯燥，我们来看几个具体的例子。需要特别注意的一点是：大部分标识字符可以同时使用。</p>

<pre>System.out.println(String.format("%1$,09d", -3123));
System.out.println(String.format("%1$9d", -31));
System.out.println(String.format("%1$-9d", -31));
System.out.println(String.format("%1$(9d", -31));
System.out.println(String.format("%1$#9x", 5689));
//结果为：
//-0003,123
// -31
//-31
// (31)
// 0x1639　</pre>

<p><strong>2.对浮点数进行格式化：%[index$][标识][最少宽度][.精度]转换方式</strong></p>

<p>我们可以看到，浮点数的转换多了一个"精度"选项，可以控制小数点后面的位数。</p>

<p>标识：</p>

<ul><li>'-' 在最小宽度内左对齐，不可以与"用0填充"同时使用
</li><li>'+' 结果总是包括一个符号
</li><li>' ' 正值前加空格，负值前加负号
</li><li>'0' 结果将用零来填充
</li><li>',' 每3位数字之间用"，"分隔(只适用于fgG的转换)
</li><li>'(' 若参数是负数，则结果中不添加负号而是用圆括号把数字括起来(只适用于eEfgG的转换)
</li></ul>
<p>转换方式：</p>
<ul>
<li>'e', 'E' -- 结果被格式化为用计算机科学记数法表示的十进制数
</li><li>'f' -- 结果被格式化为十进制普通表示方式
</li><li>'g', 'G' -- 根据具体情况，自动选择用普通表示方式还是科学计数法方式
</li><li>'a', 'A' -- 结果被格式化为带有效位数和指数的十六进制浮点数
</li></ul>
<p><strong>3.对字符进行格式化：</strong></p>

<p>对字符进行格式化是非常简单的，c表示字符，标识中'-'表示左对齐，其他就没什么了。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Helen</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Helen</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;109***2166@qq.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="http://soft.chinabyte.com/database/182/12171182.shtml">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2017-11-26)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-25487"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hunter</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;hun***es@126.com</p></div><div id="runoobvote-id-25487" data-commid = "25487" class="upvotejs"><a class="upvote"></a> <span class="count">75</span></div></div><div class="comt-main" id="div-comment-25487"><p><strong>关于 String 为啥是不可改变的</strong></p>

<p>这里可以根据 jdk 的源码来分析。</p>

<p>字符串实际上就是一个 char 数组，并且内部就是封装了一个 char 数组。</p>

<p>并且这里 char 数组是被 final 修饰的:</p>

<pre>public final class String
    implements java.io.Serializable, Comparable&lt;String&gt;, CharSequence {
    /** The value is used for character storage. */
    private final char value[];</pre>
<p>并且 String 中的所有的方法，都是对于 char 数组的改变，只要是对它的改变，方法内部都是返回一个新的 String 实例。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">hunter</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hunter</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;hun***es@126.com</p></div></span>5年前 (2018-03-25)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-25488"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hunter</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;hun***es@126.com</p></div><div id="runoobvote-id-25488" data-commid = "25488" class="upvotejs"><a class="upvote"></a> <span class="count">44</span></div></div><div class="comt-main" id="div-comment-25488"><p data-title="Java 中的字符串的加算法"><strong>java 中的字符串的加算法</strong></p>

<pre>String a = "a";
String b = "b";
String c = a + b;</pre>
<p>相当于：</p>
<pre>String c = new StringBuffer().append(a).append(b).toString();</pre>
<p>对于字符串的加运算，当编译成 class 文件时，会自动编译为 StringBuffer 来进行字符串的连接操作。</p>

<p>同时对于字符串常量池：</p>

<p>当一个字符串是一个字面量时，它会被放到一个常量池中，等待复用。</p>

<pre>String a = "saff";
String b = "saff";
String c = new String("saff");
System.out.println(a.equal(b));  // true
System.out.println(a.equal(c));  // true</pre>
<p>这个就是字符串的常量池。</p>

<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/25488">hunter</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hunter</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;hun***es@126.com</p></div></span>5年前 (2018-03-25)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-27437"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;惊梦</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;135***65476@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://blog.csdn.net/Sqirt/article/details/72765071">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-27437" data-commid = "27437" class="upvotejs"><a class="upvote"></a> <span class="count">142</span></div></div><div class="comt-main" id="div-comment-27437"><p><strong>String 类的常见面试问题</strong></p>


<p><strong>面试题一：</strong></p>
<pre>String s1 = "abc";            // 常量池
String s2 = new String("abc");     // 堆内存中
System.out.println(s1==s2);        // false两个对象的地址值不一样。
System.out.println(s1.equals(s2)); // true</pre>

<p><strong>面试题二：</strong></p>
<pre>String s1="a"+"b"+"c";
String s2="abc";
System.out.println(s1==s2);
System.out.println(s1.equals(s2));</pre>
<p>java 中常量优化机制，编译时 <span class="marked">s1</span> 已经成为 <strong>abc</strong> 在常量池中查找创建，<span class="marked">s2</span> 不需要再创建。 </p>

<p><strong>面试题三：</strong></p>

<pre>String s1="ab";
String s2="abc";
String s3=s1+"c";
System.out.println(s3==s2);         // false
System.out.println(s3.equals(s2));  // true</pre>
<p>先在常量池中创建 <strong>ab</strong> ，地址指向 <strong>s1</strong>, 再创建 <strong>abc</strong> ，指向 s2。对于 s3，先创建StringBuilder（或 StringBuffer）对象，通过 append 连接得到 <strong>abc</strong> ,再调用 toString() 转换得到的地址指向 <strong>s3</strong>。故 <span class="marked">(s3==s2)</span> 为 <strong>false</strong>。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">惊梦</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;惊梦</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;135***65476@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="https://blog.csdn.net/Sqirt/article/details/72765071">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2018-05-17)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-27438"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;惊梦</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;135***65476@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="../w3cnote/java-different-of-string-stringbuffer-stringbuilder.html">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-27438" data-commid = "27438" class="upvotejs"><a class="upvote"></a> <span class="count">87</span></div></div><div class="comt-main" id="div-comment-27438"><p><strong><a href="../w3cnote/java-different-of-string-stringbuffer-stringbuilder.html" target="_blank" rel="noopener nofollow">Java：String、StringBuffer 和 StringBuilder 的区别</a></strong></p>

<p><span class="marked">String</span>：字符串常量，字符串长度不可变。Java中String 是immutable（不可变）的。用于存放字符的数组被声明为final的，因此只能赋值一次，不可再更改。</p>

<p><span class="marked">StringBuffer</span>：字符串变量（Synchronized，即线程安全）。如果要频繁对字符串内容进行修改，出于效率考虑最好使用 StringBuffer，如果想转成 String 类型，可以调用 StringBuffer 的 toString() 方法。Java.lang.StringBuffer 线程安全的可变字符序列。在任意时间点上它都包含某种特定的字符序列，但通过某些方法调用可以改变该序列的长度和内容。可将字符串缓冲区安全地用于多个线程。</p>

<p><span class="marked">StringBuilder</span>：字符串变量（非线程安全）。在内部 StringBuilder 对象被当作是一个包含字符序列的变长数组。</p>

<p><strong>基本原则：</strong></p>
<ul><li>如果要操作少量的数据用 String ；</li><li>单线程操作大量数据用StringBuilder ；</li><li>多线程操作大量数据，用StringBuffer。</li></ul>

<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">惊梦</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;惊梦</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;135***65476@163.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="../w3cnote/java-different-of-string-stringbuffer-stringbuilder.html">&nbsp;&nbsp;参考地址</a></p></div></span>5年前 (2018-05-17)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-33933"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;阿颖</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;553***793@qq.com</p></div><div id="runoobvote-id-33933" data-commid = "33933" class="upvotejs"><a class="upvote"></a> <span class="count">57</span></div></div><div class="comt-main" id="div-comment-33933"><pre>public class Main {
    public static void main(String[] args) {
            String str1 = "hello world";
            String str2 = new String("hello world");
            String str3 = "hello world";
            String str4 = new String("hello world");
            System.out.println(str1==str2);
            System.out.println(str1==str3);
            System.out.println(str2==str4);
    }
}</pre>
<p>执行结果为：</p>
<pre>false
true
false</pre>
<p><strong>解析：</strong></p><p><span class="marked">String str1 = "hello world";</span> 和 <span class="marked">String str3 = "hello world";</span> 都在编译期间生成了字面常量和符号引用，运行期间字面常量 <strong>"hello world"</strong> 被存储在运行时常量池（当然只保存了一份）。通过这种方式来将 <strong>String</strong> 对象跟引用绑定的话，JVM 执行引擎会先在运行时常量池查找是否存在相同的字面常量，如果存在，则直接将引用指向已经存在的字面常量；否则在运行时常量池开辟一个空间来存储该字面常量，并将引用指向该字面常量。　</p>　
<p>众所周知，通过 <strong>new</strong> 关键字来生成对象是在堆区进行的，而在堆区进行对象生成的过程是不会去检测该对象是否已经存在的。因此通过 <strong>new</strong> 来创建对象，创建出的一定是不同的对象，即使字符串的内容是相同的。</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">阿颖</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;阿颖</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;553***793@qq.com</p></div></span>5年前 (2018-08-13)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-58990"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;GritHub</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;147***6963@qq.com</p></div><div id="runoobvote-id-58990" data-commid = "58990" class="upvotejs"><a class="upvote"></a> <span class="count">49</span></div></div><div class="comt-main" id="div-comment-58990"><p data-title="Java String 类中 concat() 方法和 + 号的区别">String 类中 <strong>concat()</strong> 方法和 <span class="marked">+</span> 号的区别:</p>

<pre>pubic class Demo{
    pulic satic void main(String[] args){
        String str1 = "a".concat("b").concat("c")；
        String str2 = "a"+"b"+"c";
        String str3 = "abc";
        String str4 = new String("abc");
        System.out.println(str1 == str2); //运行结果为false
        System.out.println(str1 == str3); //运行结果为false
        System.out.println(str2 == str3); //运行结果为ture
        System.out.println(str2 == str4); //运行结果为false
        System.out.println(str1.equals(str4)); //运行结果为true
    }
}</pre>
<p>执行结果为：</p>

<pre>false
false
true
false
true</pre>
<p>解析：</p>

<p>首先关于 new 出来的对象和 String s = "字符串" 的 == 执行结果为 false 就不多赘述了，因为 == 比较的是两个对象的地址值，equals() 比较的是字面值。那么 concat 方法和 <span class="marked">+</span> 号的区别在这里有体现了，我们查看concat方法的源码可以看到，它是通过复制数组在通过 char 数组进行拼接生成一个新的对象，所以地址值会有变动，而 <span class="marked">+</span> 号不会。（作者很菜，初来乍到，前辈多包涵）</p><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="../note/58990">GritHub</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;GritHub</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;147***6963@qq.com</p></div></span>2年前 (2020-11-27)</div></div></li><!-- #comment-## -->
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